1. Field of the Invention
This invention relates to an improved method and apparatus for performing wellbore telemetry operations. In one aspect, it relates to a method for establishing and maintaining a communication link between a subsurface location in a rotary drill string and a surface location.
2. Description of the Prior Art
In the drilling of oil wells, gas wells, and similar boreholes, it frequently is desirable to transmit data between subsurface and surface locations. One application where data transmission has received considerable attention in recent years is in wellbore telemetry systems designed to sense, transmit, and receive information indicative of a subsurface condition. This operation has become known in the art as "logging while drilling."
A major problem associated with wellbore telemetry systems has been that of providing reliable means for transmitting signals between the subsurface and surface locations. This problem can best be appreciated by considering the manner in which rotary drilling operations are normally performed. In conventional rotary drilling, a borehole is advanced by rotating a drill string provided with a drill bit at its lower end. Lengths of drill pipe, usually about 30 feet long, are added to the drill string, one-at-a-time, as the borehole is advanced in increments. In adapting a telemetry system to rotary drilling equipment, the means for transmitting the electric signal through the drill string must permit the connection of additional pipe lengths to the drill string as the borehole is advanced.
An early approach to the problem involved the use of continuous electric cable which was adapted to be lowered inside the drill string and to make contact with a subsurface terminal. This technique, however, required withdrawing the cable from the drill string each time a pipe length was added to the drill string. A more recent approach involves the use of special drill pipe. Each pipe section of the special pipe is provided with an electric conductor having connectors at its opposite ends. Electric continuity is maintained across the junction of two pipe sections by connectors of one section contacting a connector on the adjacent pipe section (see U.S. Pat. Nos. 3,518,608 and 3,518,609). Disadvantages of this system include the high cost of the special pipe sections, the need for a large number of electric connections (one at each joint), and the difficulty of maintaining insulation of the electric connectors at each joint.
Still another approach involves the use of cable sections mounted in each pipe section (see U.S. Pat. No. 2,748,358). The cable sections are connected together as pipe sections are added to the drill string. Each cable section is normally made slightly longer than its associated pipe section, with the result that a small amount of slack is present in the conductor string at all times.
A more recent approach involves the use of a cable maintained in a looped configuration within the drill string. The looped arrangement stores cable in the drill string and permits the cable to be extended as the drill string is lengthened. This technique normally requires the use of sheaves and other apparatus within the drill string to maintain the cable in the looped configuration. However, this arrangement presents flow restrictions within the relatively narrow opening. Moreover, care must be exercised to prevent cable entanglement, particularly for long cable loops.